He also pleaded guilty to two charges of passing an illegal firearm on to his father Ian. He will be sentenced at a date to be fixed, although the judge referred to legislation - which decrees a minimum of five years in prison for firearms offences - removes the discretion of judges.

The elder Mr Button had just been told he was suffering from terminal lung disease, while his 63-year-old wife Christine was in a care home with Alzheimer's.

The court heard he was being treated on a ward when his son Guy arrived with a bag containing the Second World War gun, other gun parts and 19 rounds of ammunition.

His son now faces between five and 10 years in prison unless his barrister can prove "exceptional circumstances".

Mr Button, who was planning to sell his house, left a suicide note addressed to police, claiming he had taken the gun to the hospital.

In it, he said: "The gun was given to me in the 1980s. When my son was clearing our loft recently he found it in a box. I thought I would hand it in to you (the police) as soon as possible, along with the ammunition.

"I brought the gun in with me when I came back in (to hospital) last week to take care of it so it was not found by the surveyor.

"I do not see any quality of life and I cannot get any answers. I love my family and I am sorry for what I have done."

Prosecuting barrister Alex Bull said: "In (police) interview, Guy Button said his father was becoming increasingly demotivated by his illness and was given a terminal diagnosis in 2008.

"He said he believed the gun was decommissioned and was not live. He said his father indicated his plan was to hand both the gun and the ammunition to police."

But Judge Richard Bray dismissed Button's initial story during the hearing on Thursday.

He said: "This sounds incredible and ridiculous that he was taking the gun into hospital so someone at the hospital could take it to the police.

"There are all sorts of implications of taking a gun into a hospital; a gun that is loaded, a gun that fires.

"I presume his father was on a ward and not in a room of his own.

"You do not need me to spell it out to you what could have happened had his father been too frail to use the gun properly. Anything could have happened in a ward.

"The defendant must have realised that, he is an intelligent man."

Luke Blackburn, defending Button, said his client now admitted he "misguidedly" gave the gun to his father.

Mr Blackburn said: "Guy Button had an instrumental role in the death of his father, whom he loved.

"When he was interviewed by police officers he was still coming to terms with the fact that although it was by his father's own hand, the major role was his own."

Referring to legislation introduced to ensure a minimum sentence of five years in prison for firearms offences, Judge Bray added: "This is a very sad case.

"This case has lots of difficulties because legislation removes the discretion of judges.

"What I have just said has been expressed by a number of other judges. Judges should be allowed to use their experience to pass sentences according to individual cases without having their powers circumscribed by unnecessary legislation."

Outside court, Detective Sergeant Rankin McCarrick said: "This is an extremely unusual case. It's a tragic case because someone has died. It's tragic because Ian Button felt he had to take this course of action.

"The fact is that it is a firearm and live ammunition in a public place and in particular a hospital. I am just glad it didn't affect other people."

A spokeswoman for the Crown Prosecution Service defended its decision not to prosecute Guy Button under the Suicide Act 1961.

In a statement, the spokeswoman said: "The CPS considered an offence under Section 2 (1) of the Suicide Act 1961; of aiding, abetting, counselling or proffering the suicide of another in relation to Guy Button, but considered there was insufficient evidence to bring a charge.

"While the evidence indicates that Guy Button's father committed suicide with a vintage handgun, which his son had provided for him, there was insufficient evidence to prove that Guy Button had any intention to do acts which he knew to be capable of assisting his father's suicide."